1. Field of the Invention
The present invention relates to a composition to form an overcoat layer for an organophotoreceptor and an organophotoreceptor employing an overcoat layer prepared from the composition. More particularly, the invention relates to a composition for forming an overcoat layer constituting an outermost layer of an organophotoreceptor to form an electrophotographic image, and an organophotoreceptor having good electrical and wear resistance by employing the overcoat layer prepared from the composition.
2. Description of the Related Art
In electrophotography, an organophotoreceptor includes a photosensitive layer formed on a conductive base and is in the form of a plate, disk, sheet, belt, or drum.
The principles of electrophotographically forming an image are briefly described below.
First, the surface of the organophotoreceptor is electrostatically uniformly charged and irradiated with a laser beam. Positive and negative charges are generated at portions into which a laser beam is irradiated and migrate to the surface. As the surface charges are neutralized, the surface potential in an exposed area is changed so that a latent image is formed.
Thereafter, when the latent image is developed with a toner, a visible image is formed on the surface of the organophotoreceptor. The formed image is transferred to the surface of a receiver, such as paper. The imaging process is repeated.
Both single layer and multilayer photoconductive elements have been used. In the single layer embodiment, a charge transport material and a charge generating material are combined with a polymeric binder, and then are deposited on the conductive base. In the multilayer embodiment, the charge transport material and charge generating material are in the form of separate layers, each of which is optionally combined with a polymeric binder and is deposited on the conductive base. Two arrangements are possible. In one arrangement (the “dual layer” arrangement), the charge generating layer is deposited on the conductive base, and the charge transport layer is deposited on top of the charge generating layer. In an alternate arrangement (the “inverted dual layer” arrangement), the order of the charge transport layer and charge generating layer is reversed.
In both the single and the multilayer photoconductive elements, the purpose of the charge generating material is to generate charge carriers (i.e., holes and electrons) upon exposure to light. The purpose of the charge transport material is to accept these charge carriers and transport them through the charge transport layer to discharge a surface charge on the photoconductive element.
In general, the photoreceptor easily wears due to friction against a toner, a roller or a cleaning blade during an imaging process, so that the thickness thereof decreases and the life thereof is shortened. Thus, an overcoat layer is coated on the organophotoreceptor.
In recent years, development of electrophotographic printers using a wet-type toner is underway, and there is an increasing demand to develop organophotoreceptors for a wet-type toner. Conventional compositions to form overcoat layers generally aim to extend a lifetime of organophotoreceptors suitable for a dry-type toner. That is to say, currently, development of compositions to form overcoat layers suitable for a wet-type toner is not yet actively being carried out.
An organophotoreceptor should not affect image quality adversely, which is caused by an increase of a charge potential and a decrease of an exposure potential or residual potential due to electrical or mechanical fatigue of the organophotoreceptor during printing through repeated charge-exposure-discharge cycles.
However, an organophotoreceptor with an overcoat layer unavoidably experiences the above-cited problem, compared to an organophotoreceptor without an overcoat layer. To avoid the problem, the thickness of an overcoat layer is restricted. If a thin overcoat layer is used, the coated layer may be easily worn due to friction between the layer and a cleaning blade in the presence of a wet-type toner, or scratches may be generated due to the toner or foreign matter, adversely affecting image quality.